1. Field of the Invention
The present invention is broadly concerned with improved methods of conducting solid acid-catalyzed, near- or supercritical heterogeneous chemical reactions such as alkylation reactions in order to improve the efficiency thereof and the tendency for the reactions to be prematurely terminated owing to coke laydown on the catalyst. More particularly, the invention is concerned with such methods wherein reaction product selectivity is enhanced by pressure-tuning the reaction together with use of particular types of solid acid catalysts having relatively narrow surface area and pore size characteristics. The invention also provides a way of substantially continuously maintaining a chemical reaction by appropriately timed catalyst regeneration cycles during the course of the reaction.
2. Description of the Prior Art
Conventional isoparaffin/olefin alkylation processes, practiced since the 1930's, convert light refinery gases to high octane number gasoline range hydrocarbons (e.g. trimethylpentane) using liquid sulfuric or hydrofluoric acid catalysts. These processes typically convert refinery gasses such as C4-C5 isoparaffins into more valuable branched chain gasoline-range C7-C9 alkylate compounds. Particularly valuable alkylates are trimethylpentanes (TMPs) and 2,2-dimethylbutane (neohexane) which are used as high-octane blending components for aviation and civilian gasolines. It is estimated that about 13% of the U.S. gasoline pool is made up of alkylates.
However, economic and environmental concerns associated with liquid acid catalyst handling, regeneration, and disposal, have spurred the search for an alternative process. Since the early 1970's, the use of solid acid catalysts as a replacement has been investigated (Corma, et al., Catal. Rev.-Sci. Eng., 35 (1993) 483), but in many instances the results have been disappointing. The reason is that solid acid catalysts tend to deactivate rapidly with time due to buildup of heavy hydrocarbons on the catalyst surface. The deactivating pathway is believed to suppress the hydride transfer mechanism, which is dependant on the acid site density (Pater, J., et al. Ind. Eng. Chem. Res. 38 (1999) 3822). Among the most common solid acids to be investigated are zeolites, sulfated zirconia, and aluminum chloride. (Corma, et al., Catal. Rev.-Sci. Eng., 35 (1993) 483; Rao, P., et al., Prep.-Div. Pet. Chem. ACS, 41 (1996) 685; and Weitkamp, J. and Traa, Y. Catal. Today, 49 (1999) 93).
U.S. Pat. No. 5,907,075 represents a significant advance in the art and describes improved solid acid catalyst supercritical alkylation processes which ameliorate the catalyst coking problem. The '075 patent teaches that use of a co-solvent or diluent such as CO2 together with supercritical reaction conditions have the effect of lessening the coke laydown difficulty. However, after a certain period of reaction time, the catalyst will nevertheless become deactivated because of coke buildup, and a “breakthrough” will occur, meaning that the production of desired reaction product will decline usually in a precipitous fashion.
The liquid-like densities yet significantly better-than-liquid diffusivities of supercritical (sc) reaction media have been shown to be more desirable than either liquid or gas phases to mitigate catalyst deactivation by coking (Subramaniam, B., Appl. Catal. 212 (2001) 199). Gas phase isobutane/butene alkylation is not practical because of the low volatility of the primary products (C8's), which undergo subsequent reaction such as oligomerization on the catalyst resulting in rapid catalyst deactivation. Operation in a liquid phase provides the maximum solubility for removing adsorbed heavy hydrocarbons. However, the pore diffusion rate in a liquid is much lower than that in a gas phase. This increases the likelihood of readsorption and further reaction of the solvated molecules in the pore.
It has been shown in the literature that isobutane (Pc=36.5 bar, Tc=408 K) /butene (PC=40.2 bar, TC=420 K) alkylation on solid acid catalysts at supercritical temperatures suffers from increased butene oligomerization and cracking reactions at these temperatures, increasing the catalyst deactivation potential (Fan, L., et al. Ind. Eng. Chem. Res., 36 (1997) 1458; Funamoto, G., et al. Res. Chem. Intermed., 24 (1998) 449; and Gayraud, P., et al. Catal. Today, 63 (2000) 223). Lower temperatures tend to favor the alkylation reaction. Supercritical operation at 95° C. can be facilitated by diluting the isoparaffin/olefin feed with suitable amounts of a low Tc inert solvent such as CO2 (Pc=73.8 bar, Tc=304 K), and has been shown to give rise to steady alkylation activity on USY and beta zeolites (Clark, M., et al., Ind. Eng. Chem. Res., 37 (1998) 1243). However, the alkylate yields are very low (<10%) on these catalysts, attributed to severe pore diffusion limitations on these catalysts.
Nafion® is a perfluorinated polymer with sulfonic acid groups grafted to side chains, yielding acidity similar to that of sulfuric acid (F{hacek over (a)}rcaiu, D. et al. J. Am. Chem. Soc., 119 (1997) 11826). Nafion® has not been extensively studied as a catalyst for isoparaffin alkylation, although it has shown good activity for a number of acid catalyzed reactions (Olah G., et al. Synthesis (1978) 672; Chaudhuri, B., et al., Ind. Eng. Chem. Res., 30 (1991) 227; Yamato, T., et al., J. Org. Chem., 56 (1991) 2089; and Sun, Q., et al. Ind. Eng. Chem. Res., 36 (1997) 5541). Nafion® is available in both unsupported and supported forms. In the supported form, the polymer is impregnated on high surface area silica supports, which has been shown to improve accessibility to acid sites (Harmer, M., et al. Chem. Comm., (1997) 1803; and Sun, Q., et al. Ind. Eng. Chem. Res., 36 (1997) 5541).
Rørvik, et al. studied unsupported Nafion® for isobutane/1-butene alkylation in a stirred liquid phase batch reactor (Rørvik, T., et al. Catal. Lett., 33 (1995) 127). The production of trimethylpentanes (the most desirable alkylate product) was shown to cease within 30 minutes of operation. More recently, silica-supported Nafion® was used to catalyze the same reaction (Botella, P., et al. J. Catal., 185 (1999) 371). Once again, rapid deactivation with respect to trimethylpentane formation was observed. It was hypothesized that the strongest acid sites—the most active for alkylation—are also the first to be poisoned.